The basic objective of this research is to determine the biological consequences of DNA damage and repair in mammalian cells caused by low level radiation and chemical exposure, particularly with regard to cell cycle variation. Our goal is to determine the biochemical basis of cell cycle-dependent variation in the cytotoxicity and mutagenicity of radiation and chemical agents, and to define the relationships of certain DNA repair processes to mammalian cell survival and mutagenesis. Although variation in the response of mammalian cells to the effects of radiation and chemicals as a function of cell cycle stage has long been recognized, the cellular mechanisms that underlie these variations in response are not understood. To approach the problem of what these mechanisms are, and how DNA repair processes are involved in cell cycle-dependent variation in response, our specific aims are: 1) To determine the extent to which specific DNA repair processes affect the expression of potentially mutagenic or lethal lesions in mammalian cells, by comparison of cell survival and mutation induction in repair-proficient versus -deficient CHO cells after single-dose, multiple low-dose, or stationary phase holding protocols, 2) To characterize cell cycle-dependent variation in the cytotoxicity or mutagenicity of radiation and various chemical agents in repair-proficient versus deficient CHO cells, in order to determine whether such phenomena reflect cell cycle-specific variations in DNA repair efficiency, 3) To utilize DNA-mediated gene transfer (DMGT), employing synchronous populations of repair-proficient and deficient CHO cells as recipients, to analyze the effects of specific types of DNA damage on the integration and expression of transfected sequences introduced at various stages of the cell cycle, 4) To clone mammalian DNA repair genes that complement the repair deficiencies in our UV-hypersensitive, DNA repair-deficient CHO cell mutants. These genes would then be used as molecular probes to study both cell cycle-specific variation in repair gene expression, and inducibility of repair gene expression by UV or chemical agents.